This invention relates to silicone resin coating compositions and methods for producing silicone resin coatings on substrates. More particularly, it relates to silicone resin coating compositions having a basic pH provided by a volatile base, such as ammonia, and methods for producing coatings therefrom. These coatings demonstrate improved resistance to ultra violet light, stress, heat and humidity.
The substitution of glass with transparent materials which do not shatter has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses and airplanes. Lenses for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch due to everyday contact with abrasives, such as dust, cleaning equipment and/or ordinary weathering. Continuous scratching and marring results in impaired visibility and poor esthetics, oftentimes requiring replacement of the glazing or lens.
Attempts have been made to improve the abrasion resistance of these transparent plastics. For example, coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium have been developed to impart scratch resistance. U.S. Pat. Nos. 3,708,225, 3,986,997, 3,976,497, 4,368,235 and 4,324,712, describe such compositions and are incorporated herein by reference.
While these aforementioned coating formulations have been found acceptable, there still remains room for improvement. For example, this invention provides coatings having an added degree of resistance to moisture, heat, humidity and ultraviolet light, which does not exist in similar coatings provided in the patents referred to above.
Coating compositions known to the art, having a basic pH, usually contain the base sodium hydroxide since the colloidal silicas utilized in these compositions are stabilized with sodium hydroxide to prevent agglomeration or gelation of the colloidal dispersion. The colloidal silicas provide excess quantities of sodium hydroxide and the pH of the coating composition must be buffered with acetic acid or other suitable acids.
Sodium hydroxide is the predominant stabilizing species for basic colloidal silica sols since most commercial processes that produce colloidal silica utilize sodium silicate as a starting material, which generates sodium hydroxide.
Those colloidal silicas which contain sodium hydroxide, such as Ludox.RTM. LS, are preferred in the coating compositions of the prior art. Sodium hydroxide is a nonvolatile base and will not vaporize from the coating composition during processing or upon application of said composition to a substrate.
It has been discovered that silicone resin coating compositions with an alkaline pH provided by a volatile base, such as ammonia, provide coatings with greater resistance to the elements than alkaline coating compositions which contain sodium hydroxide.
Not being bound by theory, it is believed that the superior properties are attributed to the reduced concentration of base and salts of the base in the cured coatings since the volatile base vaporizes from the composition during curing. Sodium hydroxide and the salts produced therefrom, such as sodium acetate, do not volatilize and remain in the cured coating. The alkaline species within the coating compositions and salts therefrom function as catalysts which promote the condensation reaction that cures the composition. It is further believed that when these alkaline species and their salts remain within the cured coating, they also catalyze hydrolysis reactions, which result in the formation of cracks in the coating under conditions of heat, humidity and ultraviolet light exposure.
All alkaline species, such as potassium hydroxide, calcium hydroxide, ammonium hydroxide and the like, including their salts, will provide catalysis for the hydrolysis reaction within the coating composition. However, where a volatile base is utilized, such as ammonia, the coating compositions can be cured under conditions so that a substantial portion of these catalysts are removed and cannot aid the formation of cracks. The coatings obtained therefrom exhibit greater resistance to humidity, heat, stress and ultraviolet light.